Reducing the cost of compressed air

06 Apr 2018

Reducing the cost of compressed air

Andy Jones

The Government’s recent 'Clean Growth Strategy’ set out its proposals for decarbonising the UK’s economy in the future whilst growing the country’s national income.  Recognising that 25 % of carbon emissions is generated by business and industry, the Government has promised to develop a package of measures to support businesses to improve their energy productivity, by at least 20% by 2030, and to work with seven of the most energy intensive industrial sectors on joint energy efficiency plans.

So how do compressors factor into this? According to a 2016 paper on the energy saving potential of existing air compressors, the electrical consumption of the industrial sector constitutes over 50 per cent of global usage, and up to 20 per cent of this can be attributed to air compression and delivery to final uses.[i] It is therefore safe to say that this is an area where energy efficiency is critical, and that compressors are a key part of lowering the overall usage of energy in industry. This is not lost on policy makers, and legislation is likely to come into play in the near future which removes inefficient compressors from the market.

However, it is at this point, where compressor efficiency is evaluated, that problems begin to arise and questions must be asked about how we analyse lifetime performance.

A whitepaper launched by Mattei this year, Air Compressors The real economic and environmental impact of using the current industry standard Life Cycle Cost analysis - has revealed the serious inaccuracies in how compressor efficiency is currently assessed, especially in respect to the difference between vane and screw technologies.

The core issue centres around the accuracy of the Life Cycle Cost (LCC) calculation. At present, this considers three factors capital equipment expenditure (CAPEX), ordinary maintenance costs and energetic consumption costs. Generally based on independently verified CAGI datasheets, this information is combined to produce an LCC for a compressor. But, as addressed in Mattei’s whitepaper, this calculation assumes that air compressor Specific Energy stays constant over time.

Specific Energy is generally based on the number of kW required to compress 1 m³/min of air. This is then multiplied by the Free Air Delivery (FAD), operating hours, and local cost of electricity, to produce the complete running cost of the compressor. However, as referenced earlier, this can only form part of an accurate LCC if we believe that this level of Specific Energy will not change over time and this is not correct for vane or screw compressors.

Screw compressors

In the case of screw compressors, efficiency deteriorates from the beginning of operation. This is essentially due to wear related to the fundamental design of this technology.

It is an accepted fact that roller bearings and thrust bearings are subject to wear, and that the rate of this is contingent on speed and load. To counter this, screw compressor manufacturers advise the substitution of all major rolling and thrust elements after a certain number of operational hours often between 40,000 and 50,000.

This wear results in a degradation of performance over time, especially as the unit approaches the point where parts need to be replaced. Although there is a surprising lack of research around this topic, there are small studies available from energy audit companies which illustrate the phenomenon. For example, one such audit, which tested 27 refrigeration screw compressors varying in age up to ten years, found that the average deterioration level was 30 per cent, with the worst compressor performing at 55 per cent degradation level.

This clearly has major implications for the LCC of a screw unit. As the standard calculation uses the zero-hour data provided by the manufacturer’s CAGI verified datasheet, it paints a very misleading picture, as it doesn’t take into account inevitable wear, and how this relates to the Specific Energy level of the unit over time. This poses a real problem, as it means that those looking to purchase a compressor, or policymakers building energy related regulations, are using an incorrect LCC, which is far removed from the real costs and efficiency of these units over their lifespan.

Vane compressors

Conversely, the efficiency of a rotary vane compressor actually improves over an initial running-in period.

The design of rotary vane compressors means that there are no roller and thrust bearings to experience wear within the unit, which means that the technical datasheet remains accurate throughout the life of the compressor, regardless of running hours. This also means that there is no need for part substitution, so rotary vane manufacturers like Mattei can offer extended warranties without a cap on operational hours.

Beyond this, from the moment a rotary vane compressor is turned on, to around the 1,000-hour mark, the blades undergo a polishing process.  This results in less friction, and consequently, better operation and reduced energy requirement. Tests conducted by Mattei in 2016 found that after 1,000 hours of operation, the two Mattei units analysed (Maxima 75 Xtreme, Maxima 55) both presented remarkable improvements in Specific Energy levels, with an enhancement of up to 5 per cent.

Given that the Specific Energy level of a screw compressor will degenerate from the beginning of operation, whilst a rotary vane will improve, it is clear that the zero-hour Specific Energy of these units should not be used as part of an authoritative LCC.

This is no small matter a 75kW screw compressor which sustains a degradation in performance of 10 per cent over ten years can cost an owner 12 per cent more, including part replacement costs, than a 75kW vane compressor which has an identical zero-hour performance, amounting to an additional cost of almost £150,000 for the user of the screw compressor.

In overlooking the changes in efficiency over time, buyers cannot see the true cost of owning and running an air compressor over its lifespan. Not only are investment decisions being made based on misleading information; the effectiveness of any potential legislation to improve the efficiencies of industrial air compression may also be compromised. That’s why it’s time to use variable Specific Energy information which reflects a compressor’s energy performance more accurately, and improve how the LCC of compressors is calculated to benefit both industry and the environment.

For further information visit www.mattei.co.uk or call 01789 450577.

i https://www.edie.net/news/11/Engineering-sector--energy-efficiency-must-play-central-role-in-Industrial-Strategy/

* Compressed Air and Gas Institute, based in North America. This body only provides information for 60Hz air compressor models, but as there is no independently verified 50Hz data published in the same format these are often referred to as a guide